Very small holes of arbitrary contours are important for many applications, including fabricating vias in printed circuit boards and integrated circuit packages. Traditional mechanical means of forming vias, such as drills or punches, are capable of forming vias as small as 0.004 inch (0.10 mm). Below this limit, lithographic or optical techniques must be used.
One of the optical techniques for micro-machining is direct ablation with a laser focused onto the workpiece with a microscope. Vias are drilled one-at-a-time with the stage on which the workpiece sits scanning to the next via to be formed, repeating the procedure until all of the vias are formed. This technique may include the use of a patterned metal mask over the surface of the substrate so that the laser ablates material only through the openings in the metal. After ablation the metal can be patterned or removed as the design requires. The processing quality of this method is limited in that it takes a relatively long time to complete a single workpiece.
Lithographic techniques include determining the pattern of vias with photoresist then subjecting the workpiece to reactive ion etch. The photoresist is subsequently removed leaving the vias patterned in the workpiece. The reactive ion etch involves the use of a very expensive piece of equipment, and several additional process steps are required for patterning, etch and photoresist strip.
It is known that laser machining can be done using modulated zone plates, holographic optical elements and binary phase gratings. These diffractive optics are more efficient than mask-based techniques, theoretically being capable of directing 100% of the incident light into a desired image. However, these diffractive optics techniques are limited in that many of them are only capable of generating a periodic matrix of spots, and they often require a refractive lens for focusing which may reduce efficiency and introduce aberrations and distortion.